ive just converted my auto (shit, just realised my sig is no longer true!) and the difference is MASSIVE. at WOT in the auto with 1st and 2nd gear having full traction to 1st, 2nd and 3rd gear having no traction. you can definitely have more fun in a manual, its much more responsive than an auto. you can have fun in an auto - but more so in a manual.
auto is much better for drags (most of the sub 9 sec cars are auto - obviously not stock though). manual is better at everything else.
the new semi-automatics you see in the more modern supercars are getting much more common and with the rev matching function (like in the 370z) they would be better for track work, but personally if i had a choice between the ~300k supercars i'd get the audi R8 because of the manual gearbox. paddle shifters are for the gays. where has the racing feel gone??